The present invention relates in general to a carbon-air fuel cell, and more particularly, to a system and a method for stabilizing a molten metal hydroxide electrolyte in a direct carbon-air fuel cell.
The use of a molten metal hydroxide as an electrolyte in a direct carbon-air fuel cell has several distinct advantages over the use of a molten metal carbonate. These advantages include higher electrical conductivity, higher electrochemical activity of carbon and oxygen electrodes, lower operating temperatures (500° C. versus 750°) and consequently use of less expensive materials for cell container. Thus, an inexpensive ultra-low carbon steel material can be used to fabricate the cell container. Furthermore, when this material is doped with 1-2% titanium, a surface oxide containing a degenerative semiconductor with stable electrical conductivity and enhanced corrosion stability is formed. Moreover, this material was found to possess excellent catalytic properties for oxygen reduction. Therefore, it can be used to fabricate both cell containers and cathodes. At the lower operating temperature of the molten metal hydroxide, the dominant product of carbon oxidation is CO2. This allows four electrons to be exchanged per one carbon atom. In a molten carbonate system at temperature higher than 750° C., the dominant product is CO where only two electrons are exchanged per one carbon atom. This allows the fuel cell using molten metal hydroxides to have a higher operating voltage than a fuel cell using molten metal carbonate electrolytes. Furthermore, in a molten hydroxide electrolyte the electrochemical activity of the oxygen/air cathode is exceptionally high. That is not the case with molten carbonate electrolyte. This enables a very simple cell design in which the oxygen electrode is not a complex high surface area structure as is used in traditional fuel cell designs. Thus, a solid carbon rod or plate immersed into molten metal hydroxide electrolyte can serve as the anode fuel. The carbon fuel can also be in form of chunks and other particulate type of carbon material. The molten hydroxide is contained in the cell container which can also act as the air cathode. The shape of the cell container can be either cylindrical or prismatic. The air cathode is fed with oxygen by introducing air into the molten electrolyte via a gas bubbler located at the bottom of the cell. The simple cell design of the carbon-air fuel cell with a molten hydroxide electrolyte makes the cost of this cell substantially lower than the cost of the cell with a carbonate electrolyte.
However, these advantages have, in the past come at a price, primarily due to the lack of invariance of the molten hydroxide electrolyte caused by its reaction with carbon dioxide produced at the anode resulting in the formation of carbonate salt. The carbonate salt adversely affects the cell operation and in the course of time lessens its efficiency.
A need therefore exists for a system and a method that reduces or eliminates the carbonization of a molten metal hydroxide electrolyte in a carbon-air fuel cell.